What information would Mt Gox give the FBI exactly that the FBI doesn't already have?
The name/username/email/hash database table is already out of the bag, so the FBI already had access to it, just like everyone else.
The only thing that Mt Gox could really give other than that would have been the exact transaction data and access logs, but if the FBI really wanted that, they could easily get that on their own. They've been blatantly monitoring internet traffic since the 90's.